Height Map Using Vertex World Height (Z) as RGB Value
Connect the depth output to a viewer node, then open an UV/Image window and select the viewer node as display.

If you click on the image (which seems generally totally white) you can read the values of every pixel.



So, you can directly save as image using a format that can handle theese "super white" values, like openEXR.

Just remember that Depth pass is not world height but distance from camera, so you will have to add a simple math node, subtracting the Depth value from the Z location of the camera (in my example 5 Blender Units)